Prophecy of Messiah: Isaiah 7

The biblical betulah (aTlvTb) usually rendered "virgin," is in fact an ambiguous term which in nonlegal contexts may denote an age of life rather than a physical state. Cognate Akkadian batultu (masculine, batulu) and Ugaritic btlt refer to "an adolescent, nubile, girl." That the woman who is so called need not necessarily be a virgo intacta is shown by the graphic account in a Ugaritic myth of the sexual relations of Baal with the goddess Anath, who bears the honorific epithet btlt (see Pritchard, Texts, 142). Moreover, in an Aramaic incantation text from Nippur there is a reference to a betulta$ (aTlvTb) who is "pregnant but cannot bear" (Montgomery, in bibl. 13:9, p. 178). The male counterpart to betulah in the Bible is often bahur (rBHY), "young man," e.g., Jeremiah 31:12 [13] and Amos 8:13 (cf. Joel 1:8, where a betulah moans for her bridegroom); and the word betulah interchanges with the somewhat synonymous age term almah (hmlA), which also describes a young woman. Thus, in Genesis 24:16, 43, Rebekah is first called a betulah and then an almah. (Exactly the same interchange of the two words appears in a Ugaritic text.)

• Almah, despite a two-millennium misunderstanding of Isaiah 7:14, "Behold a young woman [LXX: parqenos, "virgin"] shall conceive and bear a son," indicates nothing concerning the chastity of the woman in question.

The only way that the term "virgin" can be unambiguously expressed is in the negative: thus, Sumerian and Akkadian, "undeflowered," and Akkadian, "not experienced," "unopened," and "who has not known a male." The description of Rebekah (Gen. 24:16), who is first called a betulah, "young woman," and then "whom no man had known" (cf. Judg. 21:12), is similar. In legal contexts, however, betulah denotes a virgin in the strict sense (as does batultu in certain Akkadian legal contexts).- Encyclopaedia Judaica [emphasis added - CA]

Is this the only example you have of a possible interchanging of the two words?

You have not provided any evidence of such legal contexts, ...

Virginity in a woman was an asset of financial as well as moral significance: a "bride price for virgins" (mohar betulot, TvlBTY rhm), clearly higher than for non-virgins, was payable to her father for the privilege of marrying her. Biblical laws deal with litigation that may arise over the financial and moral stakes of virginity:

1. Exodus 22:15—16: A man who seduces a virgin who has not been betrothed must marry her by the payment of a bride price. If the father is unwilling to permit his daughter to marry her seducer, he must still pay her father "in accordance with the bride price for virgins." In either case the father is compensated for his monetary loss. (A similar law pertaining to the seduction of an unmarried girl is found in the Middle Assyrian Laws, A, 56 (in: Pritchard, Texts, 185), where the equivalent of the Hebrew mohar betulot, in Akkadian, sim batulti, "price for virgins," must be paid by the seducer. There, too, the father is not bound to give his daughter in marriage to the seducer. The law contains an additional clause which is absent from its biblical counterpart: "The father shall treat his daughter as he wishes, "i.e., he may punish her in any way he sees fit.)
2. Deuteronomy 22:28—29: A man who rapes a virgin who has not been betrothed must pay 50 shekels of silver, (later understood as the price of a virgin), is forced to marry her, and is deprived of all future rights of divorce, (Similarly, in the Middle Assyrian Laws, A, 55 (in: Pritchard, Texts, 185), after describing the physical status of the young woman and the various places where the offense might have occurred, the law requires the culprit, if unmarried, to pay the price for virgins, marry the girl, and forfeit rights of divorce. The father in this case, having received the monetary compensation, still has the right to marry her off to whomever he pleases. If the culprit is married, the father may choose to give his daughter in marriage to him, but it is further stipulated that the father shall take the wife of the culprit to be raped in turn!)
3. Deuteronomy 22:23—27: In the case of a man who violated a virgin who was betrothed, the place of the offense is the criterion of whether she was coerced or willingly consented (cf. Middle Assyrian Laws, A, 55 (above) and Hittite Laws, 197—8, in: Pritchard, Texts, 196). If the offense took place in town, both are stoned to death, "the girl because she did not cry for help in the town, and the man because he violated his neighbor's wife." If, however, it took place in open country, it is considered rape and the man alone is put to death. It is to be noted that as regards inviolability, a betrothed virgin is like a married woman: violation of either is a capital offense. (The Laws of Eshnunna, 26 (in: Pritchard, Texts, 162) and the Laws of Hammurapi, 130 (Pritchard, Texts, 171) may be compared; both prescribe the death penalty for the rape of virgins who are legally married.)
4. Deuteronomy 22:13—22: If a bridegroom accuses his wife of not being a virgin at the time of marriage, the girl's parents must produce evidence of their daughter's virginity before the elders at the town gate. If he has falsely defamed his wife, he is flogged, fined 100 shekels, and is deprived of all rights of divorce. The large fine befits the gravity of his accusation, which would have resulted in the execution of his bride by stoning if his charge were proven correct, i.e., if she did transgress while yet in her father's house. (For cuneiform analogues to the accusation of adultery, cf. Laws of Hammurapi, 131, 132 (Pritchard, Texts, 171), and Middle Assyrian Laws, A, 17 (Pritchard, Texts, 181).)

Esteem of the unsullied purity of the virgin is reflected in the rule of Leviticus 21:13ff. that a high priest may marry only a virgin of his clan (cf. Rashbam). Ezekiel 44:22 obliges all priests to marry virgins (or a priest's widow), but they need only be Israelites.- ibid

All things considered, it is hardly surprising that "Matthew" would pull Isaiah a bit out of context and try to wring a new meaning from it. What is surprising is that this literary sleight of hand grew to become such a cornerstone of Christendom and still has modern fundamentalists so befuddled. So let's dust off our Bibles (I like the New Revised English Bible best for clarity and the Revised Standard Version for beauty) and reread the Immanuel prophecy--in context.The setting is the Syro-Ephraimite war (ca. 734 B.C.). Wicked King Ahaz of Judah was frantic about Ephraim (another name for the northern kingdom, Israel) and Damascus (capital of Syria), which were plotting a preemptive strike. Isaiah enters, offering a sign. Ahaz demurs. Isaiah storms at him for his lack of faith and then provides a sign anyway: A male child would be born. Before this child is old enough to know to "refuse evil and choose the good," Assyria would lay waste both Samaria and Damascus (7:16 ). [This sub-prophecy, in fact, came true in 2 Kings 16:9 ; 17:5-6 .] Then, to punish Ahaz, Assyria itself, with Egypt, would arise as a far greater threat.Think about this. If Ahaz was concerned with an imminent attack from Samaria and Syria, why offer a sign that would not occur for seven centuries? To Ahaz this would be no sign at all.- A Virgin-Birth Prophecy?

Do you have documentation to back up the tweaking of translators?

I wonder if you have considered the scholarly conjecture that Matthew was unable to read Hebrew?